HIV originated from a virus found in chimpanzees. It likely entered the US in the 1970s and was originally known as GRID since it seemed to affect gay men. In the early 1980s it became apparent that HIV could infect heterosexuals as well, and it was renamed to AIDS. In 1984, scientists identified HIV as the cause of AIDS. The first drug to treat AIDS, AZT, was approved in 1987. HIV attacks and weakens the immune system, ultimately leading to AIDS if not treated. It is most often transmitted through unprotected sex or sharing needles.